1. Field of the Invention
The present invention relates to transparent or pigmented powder coating materials with crosslinkers comprising hydroxyalkylamides and polyisocyanates containing uretdione groups, which materials possess reduced susceptibility to pinholing.
2. Discussion of the Background
Powder coating materials based on triglycidyl isocyanurate (TGIC) and acid-functional polyesters produce corrosion-resistant and weather-stable powder coatings. EP 0 536 085 describes, however, how expensive processes or a relatively large and thus likewise expensive purification effort are required in order to prepare the TGIC in solid form. Moreover, TGIC is classified by the European Community as a category II mutagen, which is that it is regarded as a cause of mutations, and as of May 31, 1998 has required that it be labeled as xe2x80x9ctoxicxe2x80x9d.
Toxicologically unobjectionable and at the same time more reactive as well are xcex2-hydroxyalkylamide crosslinkers. In U.S. Pat. Nos. 4,076,917 and 4,101,606, xcex2-hydroxyalkylamides are combined with polymers having at least one carboxyl or anhydride function, in particular with polyacrylates, to form powder coating materials. U.S. Pat. No. 4,988,767 describes powder coating materials based on hydroxyalkylamides and acidic acrylate resins. EP 0 322 834 describes thermosetting powder coating materials composed of xcex2-hydroxyalkylamides and polyesters containing acid groups. These coatings with xcex2-hydroxyalkylamide crosslinker are highly weather-stable, very flexible, hard, and chemical-resistant. Especially at relatively high coat thicknesses, however, the water which is released on crosslinking leads to pinholing.
U.S. Pat. No. 4,889,890 describes hybrid powder coating materials. The acidic polymers they contain are cured with polyepoxides such as, for example, polyglycidyl ethers of aromatic phenols. In order to increase the crosslinking density, a xcex2-hydroxyalkylamide is used. These hybrid powder coating materials afford good corrosion protection but are unsuited to use outdoors because of a lack of weathering stability.
Thometzek et al. in 5th Nuremberg Congress, Congress Papers, Volume 1, 251-273 (1999) describe powder coating materials comprising a hydroxy-functional polyester resin, a hydroxyalkylamide, and a bifunctional crosslinker which in addition to blocked isocyanate groups contains carboxyl groups. These powder coating materials are suitable for obtaining matte rather than glossy coatings.
Accordingly, one object of the present invention is to provide powder coating materials which give highly glossy, flexible, light-stable and weather-stable coatings of low susceptibility to pinholing.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a transparent or pigmented powder coating material containing crosslinkers comprising hydroxyalkylamides and polyisocyanates containing uretdione groups, comprising:
a) from 20 to 99% by weight of at least one acidic polymer;
b) from 1 to 25% by weight of at least one xcex2-hydroxyalkylamide;
c) from 0.05 to 10% by weight of at least one nonaromatic polyisocyanate which contains uretdione groups and has an NCO functionality xe2x89xa71;
d) from 0 to 50% by weight of at least one pigment and/or inorganic filler;
e) from 0 to 5% by weight of at least one adjuvant or auxiliary.